1. Field of the Invention
The invention relates to acoustic transducers, and more particularly, to ultrasonic acoustic transducers having high bandwidth and sensitivity.
2. Description of the Related Art
Since the latter portion of the twentieth century, ultrasonics has developed into an important field for a wide array of applications, such as detecting flaws in engineering, imaging in medicine, and signaling in marine environments. In particular, ultrasound is widely used in the detection of objects in a medium, such as finding the floor of the ocean or underground pipes. Similarly, ultrasound may be used to identify flaws and cracks in a structure.
One of the most well known applications is medical imaging for fetal evaluation, disease detection and identification, and evaluation of internal organs and structures. Ultrasound may also be used to explore characteristics of tumors and cysts that are not disclosed by conventional imaging techniques, such as conventional X-rays. Ultrasonics further facilitates the study of heart motion and the destruction of unwanted cells. The array of ultrasound uses further extends to removing debris from objects, molding plastics, and even acoustic holography.
Many of these developments are possible due to advances in the manufacture of transducers for generating ultrasonic energy. Currently, the available frequencies extend to even the gigahertz range. Crystals of certain materials, such as quartz or other piezoelectric materials, form the foundation of most modern transducers. When an alternating electrical voltage is applied across opposite faces of such a material, the material physically oscillates at the frequency of the alternating voltage. This effect has been identified in a variety of materials.
Frequency, however, is not the only relevant characteristic. For example, medical imaging typically requires highly sensitive transducers with wide bandwidth. In addition, minimal pulse duration is desirable for optimal resolution. These objectives, however, typically conflict. Measures taken to increase the bandwidth of the transducer tend to decrease the pulse duration but diminish the sensitivity. Similarly, adjusting the configuration of a transducer to improve the sensitivity tends to diminish the bandwidth of the transducer.
As an illustrative example, the performance characteristics of a conventional transducer are shown in FIGS. 8A-D. After the transducer is well-matched to its frontal matching layers, bandwidth may only be increased by increasing the backing impedance. As the backing impedance (ZB) increases from 1.5 MRayl to 10 MRayl, the sensitivity of the transducer (Vpp) diminishes from about 1.8 V peak-to-peak to 0.85 V peak-to-peak, a loss of about 6.5 dB. In addition, the increased impedance of the backing may undesirably increase the pulse duration, as may be observed in FIG. 8B for backing impedances greater than about 6.5 MRayl. Thus, the configuration of the transducer tends to represent a compromise between competing considerations of sensitivity, bandwidth, and pulse duration.